How can I replace multiple empty lines with a single empty line in bash?

Question

I have a file that contains:

something    something else  something else again 

I need a bash command, sed/grep w.e that will produce the following output

something  something else  something else again 

In other words, I need to replace multiple blank lines with just a single blank line. grep/sed are line based. I've never found a BASH solution that would work on multi-line regex patterns.

Answer 1

For BSD-derived systems (including GNU):

You just need cat with the -s option which causes it to remove repeated empty lines from its output:

cat -s 

From man page: -s --squeeze-blank: suppress repeated empty output lines.

Answer 2

I just solved this problem by sed. Even if this is a 7 year old question, someone may find this helpful, so I am writing my solution by sed here:

sed 'N;/^\n$/D;P;D;'